Nitrogen stable isotopes in the shell of Mercenaria mercenaria trace wastewater inputs from watersheds to estuarine ecosystems

We tested the usefulness of delta N-15 values in the organic matrix of whole shells from Mercenaria mercenaria as tracers of anthropogenic nitrogen inputs to coastal ecosystems, Low and high stringency acidification methods were used to define parameters for reliable delta N-15 determination in shell material for comparison with delta N-15 values in soft tissues. delta N-15 values in shell from transplanted and native clams reflected %-wastewater contribution to estuaries, but were 2.3 to 2.5% lighter than delta N-15 values in soft tissues, Accuracy of delta N-15 values in shell material depended on recovering a sufficient quantity of organic N from shell (similar to 70 mu g) and was not altered by acidification method. Reliable delta N-15 values were obtained with as little as 80 mg of shell and using 100 mu l of acid, but higher stringency methods (treating more shell with more acid for longer duration) typically yielded more N for subsequent stable isotope analysis. Conversely, higher concentrations of acid reduced N recovery. These results suggest that the content of N recovered was of greater concern to obtaining reliable delta N-15 values from shell material than acidification effects. Differences between delta N-15 values in shell material and soft tissues likely reflected differences in N assimilation among tissues, In combination with other analyses, this method may be applied to refine modern. and historical trophic assessments and discern natural from anthropogenic influences on coastal ecosystems